RESEARCHERS who picked up and analysed wild chimp droppings said today they had shown how the AIDS virus originated in wild apes in Cameroon and then spread in humans across Africa and eventually the world. Their study, published in the journal Science, supports other studies that suggest people somehow caught the deadly human immunodeficiency virus from chimpanzees, perhaps by killing and eating them.

"It says that the chimpanzee group that gave rise to HIV ... this chimp community resides in Cameroon," said Beatrice Hahn of the University of Alabama, who led the study.

"But that doesn't mean the epidemic originated there because it didn't," said Ms Hahn, who has been studying the genetic origin of HIV for years.

"We actually know where the epidemic took off. The epidemic took off in Kinshasa, in Brazzaville."

Kinshasa is in the Democratic Republic of Congo, formerly Zaire, and faces Brazzaville, in Congo, across the Congo River.

Studies have traced HIV to a man who gave a blood sample in 1959 in Kinshasa, then called Leopoldville. Later analysis found the AIDS virus.

In people, HIV leads to AIDS but chimps have a version called simian immune deficiency virus that causes them no harm.

Humans are the only animals naturally susceptible to HIV.

AIDS was only identified 25 years ago. The virus now infects 40 million people around the world and has killed 25 million. Spread via blood, sexual contact and from mother to child during birth or breast-feeding, HIV has no cure and there is no vaccine, although drug cocktails can help control it.

And like so many new infections, AIDS appears to have been passed to humans from animals they slaughtered.

SIV has been found in captive chimps but Ms Hahn wanted to show it could be found in the wild, too.

Her international team got the cooperation of the government in Cameroon and they hired skilled trackers.

"The chimps in that area are hunted. It's certainly impossible to see them. It is hard to track them and find these materials," she said.

But the trackers managed to collect 599 samples of droppings. Ms Hahn's lab found DNA, identified each individual chimp and then found evidence of the virus.

"We went to 10 field sites and we found evidence of infection in five. We were able to identify a total of 16 infected chimps and we were able to get viral sequences from all of them," Ms Hahn said.

Up to 35 per cent of the apes in some communities were infected. Not only that, they could find different varieties, called clades, of the virus.

"We found some of the clades were really, really very closely related to the human virus and others were not," she said.

Chimps separated by a river were infected with different clades, Ms Hahn said.

And a river may have carried the virus into the human population.

"So how do you get from southern Cameroon to the Democratic Republic of Congo?" Ms Hahn said.

"Some human must have done so. There is a river that goes from that southeastern corner of Cameroon down to the Congo river."

Ivory and hardwood traders used the Sangha River in the 1930s, when the original human-to-human transmission is believed to have happened. Ms Hahn's study suggests the virus passed from chimpanzees to people more than once.

"We know that you don't get it from petting a chimp, or from a toilet seat, just like you can't get HIV from a toilet seat. It requires exposure to infected blood and infected body fluids. So if you get bitten by an angry chimp while you are hunting it, that could do it."

Ms Hahn's study only applies the HIV group M, which is the main strain of the virus responsible for the AIDS pandemic.

"It is quite possible that still other (chimpanzee SIV) lineages exist that could pose risks for human infection and prove problematic for HIV diagnostics and vaccines," her team wrote.

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